Conventionally, a threaded fastener having a hexagonal head is generally widely used. In a wrenching operation of such a threaded fastener having the hexagonal head, however, a driving angle α (see FIG. 10) is 60°, namely, there is generated a component of force which component is not directed for wrenching the threaded fastener. It is therefore difficult that the hexagonal head is made compact or reduced in its diameter and axial height. As shown in FIG. 10, the driving angle α, which is 60° in case of the hexagonal head 100, corresponds to an angle between the direction of a surface orthogonal force N and the direction of an effective wrenching force F, wherein the surface orthogonal force N corresponds to a force applied to a force-applied point Q (corresponding to a vertex of the hexagon) while the effective wrenching force F corresponds to a force effective for wrenching the threaded fastener. A wrenching torque T is expressed by an expression (1) where “F” and “r” represent the effective wrenching force and a radius of the hexagonal head, respectively. In this instance, as is apparent from an expression (2), the surface orthogonal force N applied to the force-applied point Q is twice as large as the effective wrenching force F. Meanwhile, an expansive reaction force E, whose direction coincides with a radial direction of the hexagonal head, is about 1.7 times as large as the effective wrenching force F. If the radius r of the hexagonal head requires to be reduced without reduction in the wrenching torque T, the effective wrenching force F has to be increased inversely with the reduction of the radius r. For increasing the effective wrenching force F, the surface orthogonal force N and the expansive reaction force E have to be increased. That is, it will be necessary to increase an operation force for rotating the wrenching tool, thereby causing a risk of eroding the force-applied point Q, i.e., a corner portion of the hexagonal head 100.T=6×F×r  (1)N=F/cos 60°=2F  (2)E=F×tan 60°≈1.7F  (3)
With respect to “Hexagonal head bolt” of JIS B1180 and “Hexagonal head bolt with washer flange” of JIS B1189, their dimensions s, e, k as indicated in FIGS. 11 and 12 are specifically defined in relation with a nominal diameter of thread as shown in FIG. 13. The dimension s represents a diameter of a circle inscribed in the contour of the hexagonal head. The dimension e represents a diameter of a circle circumscribed about the hexagonal contour. The dimension k represents a height of the hexagonal head. A ratio of the circumscribed circle diameter e with respect to the nominal thread diameter d is represented by “e/d”. A ratio of the height k of the hexagonal head with respect to the nominal thread diameter d is represented by “k/d”. As is apparent from FIG. 13, the circumscribed circle diameter e is not smaller than 1.55d, and the height k of the hexagonal head is not smaller than 0.6d.
The present invention was made under the above-described background with object of providing a threaded-fastener wrenching structure which makes it possible to reduce the size and weight of a threaded fastener, without reducing the wrenching torque applied to the threaded fastener.